The Constructors

​A constructor initializes an object when it is created. It has the same name as its class and is syntactically similar to a method. However, constructors have no explicit return type.

Typically, you will use a constructor to give initial values to the instance variables defined by the class, or to perform any other startup procedures required to create a fully formed object.

All classes have constructors, whether you define one or not, because Java automatically provides a default constructor that initializes all member variables to zero. However, once you define your own constructor, the default constructor is no longer used.

Example

Here is a simple example that uses a constructor without parameters −

// A simple constructor.classMyClass{int x;// Following is the constructorMyClass(){
      x =10;}}

You will have to call constructor to initialize objects as follows −

publicclassConsDemo{publicstaticvoid main(String args[]){MyClass t1 =newMyClass();MyClass t2 =newMyClass();System.out.println(t1.x +" "+ t2.x);}}

Output

10 10

Parameterized Constructor

Most often, you will need a constructor that accepts one or more parameters. Parameters are added to a constructor in the same way that they are added to a method, just declare them inside the parentheses after the constructor’s name.

Example

Here is a simple example that uses a constructor with a parameter −

// A simple constructor.classMyClass{int x;// Following is the constructorMyClass(int i ){
      x = i;}}

You will need to call a constructor to initialize objects as follows −

publicclassConsDemo{publicstaticvoid main(String args[]){MyClass t1 =newMyClass(10);MyClass t2 =newMyClass(20);System.out.println(t1.x +" "+ t2.x);}}

This will produce the following result −

Output

10 20

The this keyword

this is a keyword in Java which is used as a reference to the object of the current class, with in an instance method or a constructor. Using thisyou can refer the members of a class such as constructors, variables and methods.

Note − The keyword this is used only within instance methods or constructors

In general, the keyword this is used to −

• Differentiate the instance variables from local variables if they have same names, within a constructor or a method.

classStudent{int age;Student(int age){this.age = age;}}

• Call one type of constructor (parametrized constructor or default) from other in a class. It is known as explicit constructor invocation.

classStudent{int age
   Student(){this(20);}Student(int age){this.age = age;}}

Example

Here is an example that uses this keyword to access the members of a class. Copy and paste the following program in a file with the name,This_Example.java.

publicclassThis_Example{// Instance variable numint num =10;This_Example(){System.out.println("This is an example program on keyword this");}This_Example(int num){// Invoking the default constructorthis();// Assigning the local variable num to the instance variable numthis.num = num;}publicvoid greet(){System.out.println("Hi Welcome to Tutorialspoint");}publicvoidprint(){// Local variable numint num =20;// Printing the instance variableSystem.out.println("value of local variable num is : "+num);// Printing the local variableSystem.out.println("value of instance variable num is : "+this.num);// Invoking the greet method of a classthis.greet();}publicstaticvoid main(String[] args){// Instantiating the classThis_Example obj1 =newThis_Example();// Invoking the print method
      obj1.print();// Passing a new value to the num variable through parametrized constructorThis_Example obj2 =newThis_Example(30);// Invoking the print method again
      obj2.print();}}

This will produce the following result −

Output

This is an example program on keyword this 
value of local variable num is : 20
value of instance variable num is : 10
Hi Welcome to Tutorialspoint
This is an example program on keyword this 
value of local variable num is : 20
value of instance variable num is : 30
Hi Welcome to Tutorialspoint

Variable Arguments(var-args)

JDK 1.5 enables you to pass a variable number of arguments of the same type to a method. The parameter in the method is declared as follows −

typeName... parameterName

In the method declaration, you specify the type followed by an ellipsis (…). Only one variable-length parameter may be specified in a method, and this parameter must be the last parameter. Any regular parameters must precede it.

Example

publicclassVarargsDemo{publicstaticvoid main(String args[]){// Call method with variable args  
	   printMax(34,3,3,2,56.5);
      printMax(newdouble[]{1,2,3});}publicstaticvoid printMax(double... numbers){if(numbers.length ==0){System.out.println("No argument passed");return;}double result = numbers[0];for(int i =1; i <  numbers.length; i++)if(numbers[i]>  result)
      result = numbers[i];System.out.println("The max value is "+ result);}}

This will produce the following result −

Output

The max value is 56.5
The max value is 3.0

The finalize( ) Method

It is possible to define a method that will be called just before an object’s final destruction by the garbage collector. This method is called finalize( ), and it can be used to ensure that an object terminates cleanly.

For example, you might use finalize( ) to make sure that an open file owned by that object is closed.

To add a finalizer to a class, you simply define the finalize( ) method. The Java runtime calls that method whenever it is about to recycle an object of that class.

Inside the finalize( ) method, you will specify those actions that must be performed before an object is destroyed.

The finalize( ) method has this general form −

protected void finalize( ) {
   // finalization code here
}

Here, the keyword protected is a specifier that prevents access to finalize( ) by code defined outside its class.

This means that you cannot know when or even if finalize( ) will be executed. For example, if your program ends before garbage collection occurs, finalize( ) will not execute.

Java – Files and I/O

The java.io package contains nearly every class you might ever need to perform input and output (I/O) in Java. All these streams represent an input source and an output destination. The stream in the java.io package supports many data such as primitives, object, localized characters, etc.

Stream

A stream can be defined as a sequence of data. There are two kinds of Streams −

• InPutStream − The InputStream is used to read data from a source.

• OutPutStream − The OutputStream is used for writing data to a destination.

Java provides strong but flexible support for I/O related to files and networks but this tutorial covers very basic functionality related to streams and I/O. We will see the most commonly used examples one by one −

Byte Streams

Java byte streams are used to perform input and output of 8-bit bytes. Though there are many classes related to byte streams but the most frequently used classes are, FileInputStream andFileOutputStream. Following is an example which makes use of these two classes to copy an input file into an output file −

Example

import java.io.*;publicclassCopyFile{publicstaticvoid main(String args[])throwsIOException{FileInputStreamin=null;FileOutputStreamout=null;try{in=newFileInputStream("input.txt");out=newFileOutputStream("output.txt");int c;while((c =in.read())!=-1){out.write(c);}}finally{if(in!=null){in.close();}if(out!=null){out.close();}}}}

Now let’s have a file input.txt with the following content −

This is test for copy file.

As a next step, compile the above program and execute it, which will result in creating output.txt file with the same content as we have in input.txt. So let’s put the above code in CopyFile.java file and do the following −

$javac CopyFile.java
$java CopyFile

Character Streams

Java Byte streams are used to perform input and output of 8-bit bytes, whereas Java Characterstreams are used to perform input and output for 16-bit unicode. Though there are many classes related to character streams but the most frequently used classes are, FileReader andFileWriter. Though internally FileReader uses FileInputStream and FileWriter uses FileOutputStream but here the major difference is that FileReader reads two bytes at a time and FileWriter writes two bytes at a time.

We can re-write the above example, which makes the use of these two classes to copy an input file (having unicode characters) into an output file −

Example

import java.io.*;publicclassCopyFile{publicstaticvoid main(String args[])throwsIOException{FileReaderin=null;FileWriterout=null;try{in=newFileReader("input.txt");out=newFileWriter("output.txt");int c;while((c =in.read())!=-1){out.write(c);}}finally{if(in!=null){in.close();}if(out!=null){out.close();}}}}

Now let’s have a file input.txt with the following content −

This is test for copy file.

As a next step, compile the above program and execute it, which will result in creating output.txt file with the same content as we have in input.txt. So let’s put the above code in CopyFile.java file and do the following −

$javac CopyFile.java
$java CopyFile

Standard Streams

All the programming languages provide support for standard I/O where the user’s program can take input from a keyboard and then produce an output on the computer screen. If you are aware of C or C++ programming languages, then you must be aware of three standard devices STDIN, STDOUT and STDERR. Similarly, Java provides the following three standard streams −

• Standard Input − This is used to feed the data to user’s program and usually a keyboard is used as standard input stream and represented as System.in.

• Standard Output − This is used to output the data produced by the user’s program and usually a computer screen is used for standard output stream and represented asSystem.out.

• Standard Error − This is used to output the error data produced by the user’s program and usually a computer screen is used for standard error stream and represented asSystem.err.

Following is a simple program, which createsInputStreamReader to read standard input stream until the user types a “q” −

Example

import java.io.*;publicclassReadConsole{publicstaticvoid main(String args[])throwsIOException{InputStreamReader cin =null;try{
         cin =newInputStreamReader(System.in);System.out.println("Enter characters, 'q' to quit.");char c;do{
            c =(char) cin.read();System.out.print(c);}while(c !='q');}finally{if(cin !=null){
            cin.close();}}}}

Let’s keep the above code in ReadConsole.java file and try to compile and execute it as shown in the following program. This program continues to read and output the same character until we press ‘q’ −

$javac ReadConsole.java
$java ReadConsole
Enter characters, 'q' to quit.
1
1
e
e
q
q

Reading and Writing Files

As described earlier, a stream can be defined as a sequence of data. The InputStream is used to read data from a source and the OutputStream is used for writing data to a destination.

Here is a hierarchy of classes to deal with Input and Output streams.

The two important streams are FileInputStreamand FileOutputStream, which would be discussed in this tutorial.

FileInputStream

This stream is used for reading data from the files. Objects can be created using the keywordnew and there are several types of constructors available.

Following constructor takes a file name as a string to create an input stream object to read the file −

InputStream f = new FileInputStream("C:/java/hello");

Following constructor takes a file object to create an input stream object to read the file. First we create a file object using File() method as follows −

File f = new File("C:/java/hello");
InputStream f = new FileInputStream(f);

Once you have InputStream object in hand, then there is a list of helper methods which can be used to read to stream or to do other operations on the stream.

Sr.No.	Method & Description
1	public void close() throws IOException{} This method closes the file output stream. Releases any system resources associated with the file. Throws an IOException.
2	protected void finalize()throws IOException {} This method cleans up the connection to the file. Ensures that the close method of this file output stream is called when there are no more references to this stream. Throws an IOException.
3	public int read(int r)throws IOException{} This method reads the specified byte of data from the InputStream. Returns an int. Returns the next byte of data and -1 will be returned if it’s the end of the file.
4	public int read(byte[] r) throws IOException{} This method reads r.length bytes from the input stream into an array. Returns the total number of bytes read. If it is the end of the file, -1 will be returned.
5	public int available() throws IOException{} Gives the number of bytes that can be read from this file input stream. Returns an int.

There are other important input streams available, for more detail you can refer to the following links −

• ByteArrayInputStream

• DataInputStream

FileOutputStream

FileOutputStream is used to create a file and write data into it. The stream would create a file, if it doesn’t already exist, before opening it for output.

Here are two constructors which can be used to create a FileOutputStream object.

Following constructor takes a file name as a string to create an input stream object to write the file −

OutputStream f = new FileOutputStream("C:/java/hello") 

Following constructor takes a file object to create an output stream object to write the file. First, we create a file object using File() method as follows −

File f = new File("C:/java/hello");
OutputStream f = new FileOutputStream(f);

Once you have OutputStream object in hand, then there is a list of helper methods, which can be used to write to stream or to do other operations on the stream.

Sr.No.	Method & Description
1	public void close() throws IOException{} This method closes the file output stream. Releases any system resources associated with the file. Throws an IOException.
2	protected void finalize()throws IOException {} This method cleans up the connection to the file. Ensures that the close method of this file output stream is called when there are no more references to this stream. Throws an IOException.
3	public void write(int w)throws IOException{} This methods writes the specified byte to the output stream.
4	public void write(byte[] w) Writes w.length bytes from the mentioned byte array to the OutputStream.

There are other important output streams available, for more detail you can refer to the following links −

• ByteArrayOutputStream

• DataOutputStream

Example

Following is the example to demonstrate InputStream and OutputStream −

import java.io.*;publicclass fileStreamTest {publicstaticvoid main(String args[]){try{byte bWrite []={11,21,3,40,5};OutputStream os =newFileOutputStream("test.txt");for(int x =0; x < bWrite.length ; x++){
            os.write( bWrite[x]);// writes the bytes}
         os.close();InputStreamis=newFileInputStream("test.txt");int size =is.available();for(int i =0; i < size; i++){System.out.print((char)is.read()+"  ");}is.close();}catch(IOException e){System.out.print("Exception");}}}

The above code would create file test.txt and would write given numbers in binary format. Same would be the output on the stdout screen.

File Navigation and I/O

There are several other classes that we would be going through to get to know the basics of File Navigation and I/O.

• File Class

• FileReader Class

• FileWriter Class

Directories in Java

A directory is a File which can contain a list of other files and directories. You use File object to create directories, to list down files available in a directory. For complete detail, check a list of all the methods which you can call on File object and what are related to directories.

Creating Directories

There are two useful File utility methods, which can be used to create directories −

• The mkdir( ) method creates a directory, returning true on success and false on failure. Failure indicates that the path specified in the File object already exists, or that the directory cannot be created because the entire path does not exist yet.

• The mkdirs() method creates both a directory and all the parents of the directory.

Following example creates “/tmp/user/java/bin” directory −

Example

import java.io.File;publicclassCreateDir{publicstaticvoid main(String args[]){String dirname ="/tmp/user/java/bin";File d =newFile(dirname);// Create directory now.
      d.mkdirs();}}

Compile and execute the above code to create “/tmp/user/java/bin”.

Note − Java automatically takes care of path separators on UNIX and Windows as per conventions. If you use a forward slash (/) on a Windows version of Java, the path will still resolve correctly.

Listing Directories

You can use list( ) method provided by File object to list down all the files and directories available in a directory as follows −

Example

import java.io.File;publicclassReadDir{publicstaticvoid main(String[] args){File file =null;String[] paths;try{// create new file object
         file =newFile("/tmp");// array of files and directory
         paths = file.list();// for each name in the path arrayfor(String path:paths){// prints filename and directory nameSystem.out.println(path);}}catch(Exception e){// if any error occurs
         e.printStackTrace();}}}

This will produce the following result based on the directories and files available in your /tmpdirectory −

Output

test1.txt
test2.txt
ReadDir.java
ReadDir.class